The present invention relates to transmitting and receiving antennas, and, more particularly, to mobile C.B. antennas equipped with loading coils.
Antennas of the simple rod type are most efficient when they have a length equivalent to the wavelength of the signal they are to receive or transmit. Citizen band (C.B.) radio transmissions, which have become extremely popular in recent years, are conducted at the 27 MHz channel, i.e. an 11 meter or approximately 36 foot wavelength. Naturally it would be impractical to have such a long antenna mounted on a vehicle. However, it is well known that a rod type monopolar antenna is most efficient when it is equivalent to a quarter wavelength and counterposed against another quarter wavelength section or a metallic surface (e.g. a vehicle body) which acts as an additional quarter wavelength. While a quarter wavelength antenna is much more acceptable, it is still a bit too long for most vehicles and it is the practice to make C.B. antennas in the range of 3 to 4 feet in length. When this is done the impedance of the antenna as seen by the transmitter-receiver (transceiver) is altered.
For maximum power transfer from the transceiver to the antenna, the output impedance of the transmitter section of the transceiver, the characteristic impedance of the connecting cable and the impedance of the antenna should all be matched and should appear to be resistive. The degree of mismatch and, hence, the efficiency of the antenna system is given by the voltage standing wave ratio (VSWR). When the ratio is 1:1 all of the output power of the transmitter section is delivered to the antenna. However, a VSWR of some other value indicates that some of the transmitter power is reflected from the antenna back to the transmitter and is never radiated.
In order to compensate for the capacitive reactance of a short C.B. antenna, which produces an impedance that differs from the ideal impedance and results in a poor VSWR, an inductive loading coil is included in the antenna system. This coil cancels the capacitive reactance and makes the antenna appear to have the proper impedance, i.e. electrically the antenna appears longer.
A typical loading coil is in the form of conducting wire that is carefully wrapped about a form or core. The ends of the wire are then soldered or fastened, e.g. by means of screws, onto the other conducting parts of the antenna. Such a loading coil is shown in U.S. Pat. No. 2,719,920 to G. R. Ellis. A loading coil without a core, but whose turns are covered with a vinyl jacket, is disclosed on page 39 of C.B. Radio Antennas by David E. Hicks. In either case considerable effort is expended to get and maintain the correct number of turns with the right spacing for these coils. Although these coils have the desired affect of cancelling the capacitance reactance, it has been found that their high Q limits the band width of the antenna. Also, they are relatively expensive and the connection of the coil to the rest of the antenna requires a good deal of labor and represents an additional expense.
Simple springs, while not designed to be inductors, nevertheless do possess some inductance and have been used as such. In particular, U.S. Pat. No. 2,982,964 to R. H. Bresk et al. discloses a receiving antenna system that uses a tapered multi-turn spring which can be compressed to change its physical length and inductance, thereby tuning the receiving antenna. The spring is made of spring wire and in one embodiment its upper end is welded to a whip which passes through its center. In another version the spring is enclosed in a conducting housing, so that it cannot radiate, and the receiving antenna connection extends into this housing. U.S. Pat. No. 3,624,662 to A. Feder describes the use of a coil spring compressed within a dielectric sleeve as a phasing coil for a mobile radio transmitting and receiving antenna, i.e. the spring acts as a delay line so that the current distribution between the upper and lower sections of the antenna can be matched. Although one end of the spring freely rests against a conducting member at the top of the antenna, the other end is firmly attached to a sleeve.
A less expensive antenna, capable of transmitting in the C.B. band, could be designed if a method were found for using simple springs as the transmission loading coil. Even more material and labor savings could be realized if this spring could be installed in the antenna without any solder, weld or the like type of connection between the spring and the rest of the antenna.